Turbines generally include a compressor section, one or more combustors, a fuel injection system and a turbine section. Typically, the compressor section pressurizes inlet air, which is then turned in a direction or reverse-flowed to the combustors, where it is used to cool the combustor and also to provide air for the combustion process. In a multi-combustor turbine, the combustors are generally located in an annular array about the turbine and a transition duct connects the outlet end of each combustor with the inlet end of the turbine section to deliver the hot products of the combustion process to the turbine.
There have been many developments in the design of combustors as a result of continuing efforts to reduce emissions, for example, NO.sub.x and CO emissions. Dual-stage combustors have been designed in the past, for example, see U.S. Pat. Nos. 4,292,801 and 4,982,570. Additionally, in U.S. Pat. No. 5,259,184, of common assignee herewith, there is disclosed a single-stage, i.e., single combustion chamber or burning zone, dual-mode (diffusion and premixed) combustor which operates in a diffusion mode at low turbine loads and in a premixed mode at high turbine loads. In that combustor, the nozzles are arranged in an annular array about the axis of the combustor and each nozzle includes a diffusion fuel section or tube so that diffusion fuel is supplied to the burning zone downstream of the nozzle and a dedicated premixing section or tube so that in the premixed mode, fuel is premixed with air prior to burning in the single combustion zone. More specifically, and in that patent, there is described diffusion/premixed fuel nozzles arranged in a circular array mounted in a combustor end cover assembly and concentric annular passages within the nozzle for supplying fuel to the nozzle tip and swirlers upstream of the tip for respective flow of fuel in diffusion and premixed modes.
It has been discovered, however, that during the transition from the diffusion mode to the premixed mode, the combustor displays a tendency to become unstable and generates high amplitude combustion noise. Additionally, as air flow and fuel flow are varied to the combustor as required by the turbine's operating cycle, the combustor's stability and noise level can be adversely affected. The consequence of a combustor with insufficient stability is the limited turndown of the combustor. The consequences of a combustor with unacceptably high noise levels are premature wear or high cycle fatigue cracking of structural components in the combustor. To design a dry low NO.sub.x combustor, it will be appreciated that NO.sub.x emissions, CO emissions, combustion dynamics and combustion stability are factors which must be considered from an aerodynamic standpoint. The nature of the combustion process provides an interdependency of these factors.
In the combustor design disclosed in U.S. Pat. No. 5,259,184, previously discussed, fuel transfer from diffusion mode to premixed mode is effected simultaneously. That is, the fuel is transferred from all diffusion nozzles simultaneously to all premixed nozzles. To accomplish that, the fuel transfer is made by simply redirecting fuel from the diffusion supply manifold to the premixed supply manifold. While the fuel nozzle end cover was internally manifolded with a fuel supply flange feeding an internal manifold for four of five premix nozzles and a fuel supply flange feeding an internal manifold for the fifth premix nozzle, the manifold arrangement was provided only in order to cope with a generator trip event while operating in a premixed mode. All premix nozzles were intended to flow equal rates of fuel into the combustor. Thus, combustion stability and combustion dynamics created certain difficulties in using this single-stage type combustor, particularly during transition from the diffusion mode to the premixed mode.
In an effort to cure these problems, as described in a companion application Ser. No. 08/258,112, now U.S. Pat. No. 5,551,228, the supply of fuel during transition from the diffusion mode to the premixed mode is staged, as well as during steady-state operation in the premixed mode. That is, the control of the flow of fuel through the nozzles is varied to provide an asymmetric flow of fuel across the combustor during the transition from diffusion mode of operation to premixed mode of operation and during portions of the premixed mode of operation. In that invention, at the start of transition, one of the five premixed nozzles is actuated to flow premixed fuel to the single combustion zone, while the remaining four nozzles continue operation in a diffusion mode. Thus, the percentage of total fuel supplied the combustion zone by the fifth nozzle is greater than the percentage of total fuel supplied by any one of the diffusion nozzles, thus affording an asymmetrical fuel loading across the combustor. At higher loadings, fuel from the diffusion manifold is shut down and fuel is supplied to the four remaining premixed nozzles from the premix manifold. During this transition, the fifth nozzle also has a higher fuel/air ratio than any one of the remaining four premixed nozzles. Hence, the fifth nozzle runs rich and stable and stabilizes the remaining four nozzles. At full load, the fuel split among the various nozzles in the premix mode is equal. Thus, by unequally fueling the nozzles during transition and during premix operation below full load, severe combustion dynamics, i.e., high acoustical noise and resonation, is inhibited as a result of the imbalance of fuel across the combustor.
However, it has also been discovered that by varying the fuel/air ratio among the nozzles across the combustor, there is a tendency to increase NO.sub.x and CO emissions. It will be appreciated that in an annular array of nozzles about the axis of the combustor, the lowest emission levels are obtained when all of the premixed nozzles are supplied with equal, or very close to equal, amounts of fuel, with each nozzle being provided a percentage of the total fuel corresponding to the percentage of the one nozzle to the total number of nozzles. The decreases in dynamic pressure levels achieved by generating unequal fueling of the nozzles across the combustor have been found to be offset by increases in NO.sub.x and/or CO emissions over various parts of the load range. Thus, while the asymmetrical fueling of the nozzles across the combustor generates lower dynamic pressure levels, it does so at the expense of increased emissions in the premixed operating mode.